Conventionally, an electric double-layer capacitor having polarized electrodes containing steam activated carbon is known. In this electric double-layer capacitor, steam activated carbon particles are low in conductivity, so that a predetermined conductivity is maintained by adding 2 to 20 mass % of conductive filler (conductive additive) to the polarized electrodes. In this electric double-layer capacitor, the capacitance is further improved, however, recently, improvements in capacitance have reached respective limits. Therefore, an electric double-layer capacitor using alkali activated carbon for a polarized electrode instead of steam activated carbon has been proposed (for example, refer to Japanese Published Unexamined Patent Publication No. 2004-47613). The alkali activated carbon of a graphitizing carbon material has excellent conductivity due to its low porosity and graphitic structure in comparison with steam activated carbon, so that the amount of the conductive filler (conductive additive) to be added to the polarized electrode can be reduced. In comparison with steam activated carbon, the alkali activated carbon has greater ion adsorption to the steam activated carbon due to its pore distribution being sharp and small pore volume, and the capacitance density of the polarized electrode can be increased.
However, the polarized electrode using alkali activated carbon has great ion adsorption, so that the concentration gradient of ions inside the electrode body at the time of discharge is small, and therefore, the diffusion resistance increases. Further, due to expansion of alkali activated carbon when charging, the electrode body expands. As a result, there is a possibility that the gap between activated carbon particles inside the electrode body is limited and hinders ion diffusion inside the electrode body, and the internal resistance increases. Therefore, although the alkali activated carbon has higher conductivity than that of steam activated carbon, the ion diffusion resistance increases, and accordingly, the internal resistance increases. That is to say, the polarized electrode using alkali activated carbon increases in internal resistivity (Ω·cm2) per a facing electrode area compared to the polarized electrode of the same size using steam activated carbon.
Therefore, an object of the present invention is to provide a polarized electrode with which an electric double-layer capacitor whose internal resistance is small while maintaining excellent capacitance can be manufactured, and the electric double-layer capacitor.